1. Field of the Invention
The present invention relates to a steering angle detector, and more particularly to a steering angle detector for use as a front wheel steering angle detector in a four wheel steering system.
2. Description of the Relevant Art
Four wheel steering systems for turning rear wheels in relation to the turning of front wheels have a steering angle detector for obtaining front wheel steering angle information to control the turning angle of the rear wheels. As disclosed in Japanese Laid-Open Patent Publication No. 56-146670, a conventional steering angle detector comprises a potentiometer, an encoder, or the like mechanically coupled to a steering shaft for obtaining steering angle information pertaining to the angle through which a steering wheel is turned.
The mechanism for transmitting steering power from the steering wheel to the dirigible wheels suffers mechanical backlashes, twists, and other distortions, and the steering wheels have a certain play. Therefore, the turning angle of the steering wheel does not accurately correspond to the actual turning angle of the dirigible wheels, and it is difficult to obtain accurate steering angle information from the turning angle of the steering wheel.
On the other hand, with a detector that is mechanically coupled to a component of the steering system tends to malfunction due to vibrations or shocks applied to the detector by the component, or to be adversely affected by dust or environmental changes such as temperature changes.
Generally in four wheel steering systems, a gear box for turning the rear wheels is mounted on a vehicle body through a highly rigid support structure, and a gear box for turning front wheels is elastically mounted on the vehicle body in order to absorb vibration or external forces from the front wheels to the steering wheel. This is because vibration or external forces from the rear wheels are not transmitted to the steering wheel in a four wheel steering system in which the rear wheel turning gearbox is mechanically independent of the front wheel turning gearbox and the rear wheels are turned by an actuator in relation to the turning of the front wheels. It is also because the action for turning the rear wheels is required to be highly accurate with respect to the vehicle body as the turning angle of the rear wheels is smaller than the turning angle of the front wheels.
During an initial period of the front wheel turning action, the front wheel turning gearbox is moved, together with a rod member, transversely of the vehicle body because the front wheel turning gearbox is elastically mounted on the vehicle body. Where a steering angle detector is assembled in the front wheel turning gearbox, therefore, the steering angle detector may fail to detect a steering angle regardless of the fact that the front wheels are actually turned.